Preservatives are well known to significantly prolong the service life of wood and thereby extend forest resources and enhance their sustainability. For many years, chromated copper arsenate (CCA) has been widely used as a wood preservative. In this use, it appears that copper functions as a fungicide, arsenic functions as an insecticide and co-fungicide, and hexavalent chromium helps bind arsenic and copper to the wood. During the process of CCA fixation, it is believed that the hexavalent chromium is reduced to trivalent chromium, and copper and arsenic are precipitated within the wood as low soluble complex compounds.
Demand for CCA-treated wood products has increased since the early 1970's for residential applications such as decks and fences, and for industrial products such as utility poles, timbers, and marine pilings. During 2001, approximately 150 million pounds of CCA preservatives were used in the production of preservative-treated wood. About 28.8 million pounds of CCA (dry oxide concentrate basis) were consumed by the US wood treating industry in 2004. However, in 2002, the US and Canadian wood preservation industry agreed to voluntarily withdraw the use of CCA-treated wood for residential and consumer products by the end of 2003. CCA-treated wood continues to be used in industrial applications (e.g., utility poles, timbers, and marine pilings). The amount of spent CCA-treated wood is expected to increase from 3-4 million m3/yr. currently to about 12 million m3/yr. during the next 15 years. Since the expected average service life of CCA-treated wood is between 20-40 years, spent CCA-treated wood is expected to continue to be disposed for decades.
The environmental and human health impacts of CCA-treated wood have received attention because of the toxicity of copper, chromium, and arsenic. CCA-treated wood has been disposed primarily in construction and demolition (C&D) debris landfills or in municipal solid waste (MSW) landfills. Thus, there is increased concern about contamination of soil and groundwater with copper, chromium, and arsenic from disposal sites.
Alternative waste management strategies and technologies, other than landfills, for spent CCA treated wood have been developed.
Reuse Without Chemical Treatment
Because a CCA-treating solution does not penetrate the heart wood of large dimension treated products, untreated portions of treated wood products can be obtained by re-sawing or shaving the treated exterior and then re-using the heart wood. The option of re-sawing treated wood appears to be feasible for industrial products, but of limited potential for residential products. Further, mechanical abrasion of the treated exterior may cause fine wood particles contaminated with metals to become airborne, which may raise environmental concerns.
Spent CCA-treated wood may be reused in wood based composites, wood-cement composite, or other composite materials. The CCA-metals, however, tend to interfere with adhesives, and thus adversely affect the physical and mechanical properties of such composites.
Solvent Extraction of CCA
Hot sulfuric acid, hot nitric acid, aqueous ammonia solutions, acetic acid, and formic acid may be used to extract CCA. Such extraction techniques appear to convert the metals into water soluble salts. Solvent extraction, however, typically is not complete for all three CCA metals. In addition, strong acids also tend to cause excessive decomposition of the wood components.
Lianzhen et al. (U.S. Pat. No. 7,160,526) discloses a method of recovering CCA by liquefying the treated wood at high temperature with a strong acid, followed by precipitation of the metals or complexation of the metals. This method requires a temperature between about 100° C. to 250° C., which may cause vaporization of arsenic. This method also may require several hours to obtain reasonable extraction.
U.S. Pat. Application No, 2006/0292309 discloses a method to detoxify treated wood by using an aqueous solution of an alkali metal mono- or di-carboxylate as an extractant.
Another extraction method uses chelating agents, such as ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), oxalic acid, or citric acid to remove reactive metals.
U.S. Pat. Application No. 2007/0036699 discloses use of a lixivant, for example, a chelating agent, to extract Cu, Cr, and As from treated wood. There are several disadvantages for detoxifying spent CCA-treated wood products using chelating agents, including, large amounts of chemical solvent required, long extraction times typically required, and resulting extraction solutions that are difficult to handle.
Bioremediation
Bioremediation, comprising microbial metal removal from toxic wastes, has been used to extract (CCA from spent treated wood. Several varieties of brown-rot fungi have been used to remove metals from CCA-treated wood. It is believed that oxalic acid, which is thought to act as a leaching agent, is secreted from the fungi. Some metal-tolerant bacteria are effective in removing 93% Cu and 45% As, but are ineffective for removing Cr. Bioremediation also is limited by the time-consuming nature of the process and by the high cost of the nutrient culture medium.
While detoxification technology has focused primarily on removing CCA from the wood, few methods have included recycling of treated wood or recovering chromated copper arsenate for reuse.
There remains an unfilled need for a method for detoxifying CCA-treated wood that is economically effective, environmentally sound, provides for recycling spent treated wood, and allows reuse of recovered chromate copper arsenate.
We have developed a method for detoxifying CCA-treated wood that comprises microwave-enhanced acid extraction of CCA. This method allows removal of over 99% of all these metals, allows detoxified wood to be used as a base material for forming polymeric materials, and allows reuse of recovered chromated copper arsenate for treatment of untreated wood.